1. Introduction
The following description includes information that may be useful in understanding the present invention. It is not an admission that any such information is prior art, or relevant, to the presently claimed inventions, or that any publication specifically or implicitly referenced is prior art.
2. Background
Basic to the operation of modern machinery is the transmission of mechanical energy from source locations to points of utilization by means of rotating shafts transmitting torque. Beyond the nominal torques being transmitted, torque variations are ubiquitous in the rotating parts of machines. In addition to variations reflective of the actual function of the machine (e.g., impulse wrenches, rock crushers, etc.), major torque excursions also originate from kinematic features (e.g., oscillating or reciprocating parts), or inconstant rates of energy input or usage (e.g., as in piston engines, air compressors, etc.). Torque variation with rotational position arises in electrical machines from the needed presence of winding slots (“cogging” in machines having permanent magnet rotors) and commutation (“ripple” in brushless motors). Some sources of variational torque, such as those which arise on, or from the rotation of, ship propellers and helicopter rotors, are more subtle in that they arise simply from the variation with rotational position of the blade's proximity to the hull or fuselage. Moreover, all such active variational torques can stimulate torque oscillations at frequencies dependent on the dynamic relationship between inertia and elasticity within individual or interconnected rotating parts. Since the following factors mirror their causes: (i) the amplitude of torque variation, (ii) the spectrum of torque variation, (iii) timing of events affecting torque variation, and (iv) phase of torque variation relative to rotational position, it follows that sensing and measurement of only the variational torque components can often provide more detailed information concerning machine function than might be apparent from a measurement of torque alone, which is often dominated by its larger, more steady state components. This benefit is analogous to that obtained by using an accelerometer in an airbag sensor system to detect an impending crash, as opposed to a vehicle speed sensor which would not be able to detect a collision rapidly enough in the first place, and sometimes not at all due to the insufficient sensitivity to rapid velocity changes that an accelerometer is able to detect but a vehicle speed sensor is not.
In analogous fashion, torque variation can be fully characterized by its rate of change (ROC). The ROC signal has no features that directly reflect either rotational speed or torque magnitude. Continuous measurement of ROC in or of torque can provide sufficient information to identify specific torque producing events, recognize “signature” abnormalities, and signal the need for controlling action. While ROC information can, in principle, be obtained by differentiating a torque dependent signal, such an approach would fundamentally limit signal quality and frequency response in at least two ways. First, the frequency response of the base torque signal would be limited by the frequency response of the torque sensor system used. Second, the amplitude of variational torque measurable in the torque signal would be limited due to electrical noise present in the torque sensor signal or be hidden within the resolution of the torque sensor signal. Limitations of existing torque sensing technologies, such as frequency response limitations and the difficulty of obtaining an adequate resolution across the full range of torque measurement, have precluded their practical use for detecting rapid and/or small torque variations. Moreover, direct measurement of ROC provides greater sensitivity to rapid and/or small torque variations, thereby exposing detail that could be lost in the frequency response or noise limitations of more complex and more costly torque sensors.
Today, a variety of methods and devices exist for measuring torque on rotating shafts. These include elastic torque sensor systems which measure strain by use of strain gauges or phase shift measurement by way of angular position sensors but whose frequency response is generally limited to approximately 1 kHz at best, making their usage for detecting torque transients rather limited. Torque sensing methods further include magnetoelastic torque sensor systems that measure stress by monitoring the variation of stress-dependent magnetic properties such as permeability or magnetization. Such systems have either limitations on the frequency response or sensitivit/resolution of the measurement of the pertinent magnetic property, limiting their ability to detect small but rapid variational torque components.
It will become apparent in what follows that the ROC systems of the invention overcome such problems, in that:                (1) the frequency response of an ROC system is much greater than that of any existing torque sensor system;        (2) the detectable amplitude of a given variational torque is higher for higher frequencies in an ROC signal than that which can be practically obtained through a differentiated torque sensor signal        (3) small rapid changes in torque representing potentially very important diagnostic or control data, when observed in a torque sensor signal, are either not easily observable by measuring torque alone or not at all, but are detectable by measuring ROC in torque directly; and        (4) restrictions on types of transducer steels having magnetoelastic properties useful for use as the member carrying or subjected to a time-varying torque are far less stringent for ROC sensor systems than for magnetoelastic polarized band torque sensor systems, thereby providing ROC sensors with the combined benefits of high frequency response, ease of implementation, and lower cost.ROC sensors for sensing torque variation provide the following additional benefits and advantages: they require no excitation power, they can withstand very high temperatures, and they are robust.        